We use Happy Campers when we treat, which is not all the time. It’s just a holding tank, so I would clean thoroughly and let it dry out-nothing for maggots or flies to survive on. Most likely an anomoly, not something like sewage flies in a home septic system. My 2 cents.
Buce, Kathryn & Oscar the pug
Bigfoot 25 RQ
Towed by 2018 Chevy High Country Duramax